1. Field of the Invention
The present invention relates to a nickel-free grey gold alloy comprising 75-76% by weight of Au and between 5 and 14% by weight of Pd.
2. Description of the Prior Art
Problems associated with the allergy caused by nickel have led to the presence of nickel in white or grey gold alloys being reduced or even prohibited. In addition, these alloys are excessively hard and not very deformable so that they do not lend themselves well to work in particular in the fields of jeweler and watchmaking.
A nickel-free grey gold alloy having good deformability has already been proposed in CH-684,616, this alloy generally comprising, in this case, essentially between 15% and 17% by weight of Pd, between 3 and 5% of Mn and between 5 and 7% by weight of Cu. Pd is a very expensive metal, the cost of which fluctuates enormously. Lowering the proportion of Pd of the abovementioned alloy and adding Ag thereto result in a low deformability. Furthermore, too high a percentage of Ag causes the alloy to tarnish.
Moreover, JP-A-90/8160 has disclosed a ternary grey gold alloy with more than 10% by weight of Pd and more than 10% by weight of Cu, the amounts of Pd and Cu being the same, which means that the higher the Pd content the more the copper content increases, and vice versa. This amounts to saying that, for an 18 ct alloy, the respective Pd and Cu contents may only be 12.5% respectively. Furthermore, such a ternary alloy does not have the moulding properties allowing it to be used, in particular, with the so-called lost-wax technique.
The object of the present invention is to substantially improve white or grey gold alloys, allowing the proportion of Pd to be reduced without reducing its deformability properties, as well as its metallurgical properties allowing it to be used in lost-wax casting techniques.
For this purpose, the subject of this invention is a nickel-free grey gold alloy as described below.
Surprisingly, it has been found that it is possible to limit, or even reduce substantially, the proportion of Pd without impairing either the whiteness of the alloy or its metallurgical and mechanical properties, which may even be improved, by a substantial increase in the proportion of Cu. It has even been possible to show that the less Pd used the more the proportion of Cu can be increased without impairing either the colour or the desired deformability properties.
Furthermore, the incorporation of ferrous metals is also avoided so that the alloy can be used with conventional casting techniques in making jewelry and watches, as well as in the art of making dental prostheses, in which the so-called lost-wax technique is used, this being most advantageous in the case of short runs or even in the production of one-off components.
Certain other elements are added to the main elements of this alloy in order to improve its metallurgical properties, in particular to lower its melting point, to improve the grain fineness and to avoid porosity.